1. Field of the Invention
In at least one aspect, the present invention relates to covalently linked organic networks and methods for forming covalently linked organic networks.
2. Background Art
The formation of covalently linked organic networks has been an elusive goal and an attractive challenge in both molecular design and organic chemistry. These networks can be defined as periodic especially “2-D or 3-D” materials composed of strong, kinetically inert, covalent bonds (e.g. between C, O, N, B). In addition to its stimulating synthetic challenge, properties of these new materials may have important industrial applications taking advantage of their lightweight, inexpensive starting materials, and potentially high chemical and thermal stabilities. By employing specific organic units in a periodic array at the molecular scale, one can specifically tailor structure, functionality, and material properties. For this to be achieved, one needs to operate under mild conditions that do not destroy the structural or physical properties of the building blocks that may be translated into the extended network.
Covalently linked organic networks differ from existing cross-linked polymers and other polymeric materials whose properties are a result of various processing techniques in that organic crystalline networks have clearly defined molecular architectures that are intrinsic to the material. Accurate control over the position of selected organic units in an extended structure is needed to allow optimum exploitation of the material properties.
Existing crystalline covalently linked materials such as diamond, graphite, silicon carbide, carbon nitride, and boron nitride are formed under very high pressures (1-10 GPa) or very high temperatures (500-2400° C.). These extreme synthetic conditions limit the flexibility needed in the formation of extended or functionalized structures, since the structural or chemical integrity of many organic monomer units is not preserved under these conditions.
Current attempts towards synthesizing covalent networks under mild conditions have been unsuccessful in producing extended materials that have periodic molecular structures with long-range order. One such attempt involved the pre-organization of organic moieties via hydrogen bonding or metal-ligand interactions prior to the diffusion of a reactive non-metallic cross-linking agent into the channels. This linked the pre-arranged organic molecules together, and the metal template ions were subsequently removed. Incomplete polymerization or loss of crystallinity upon removal of the metal template ions, however, is often observed.
There has been an increasing demand for porous materials in industrial applications such as gas storage, separations, and catalysis. Some advantages of using completely organic porous materials as opposed to their inorganic or metal-organic counterparts, are that organic materials are lighter in weight, more easily functionalized, and have the potential to be more kinetically stable. In addition, there are environmental advantages to employing extended structures without metal components.
Some current methods of inducing porosity within polymers involve various processing methods or preparation from colloidal systems. All glassy polymers contain some void space (free volume), although this is usually less than 5% of the total volume. It is possible to “freeze-in” up to 20% additional free volume for some glassy polymers with rigid structures by rapid cooling from the molten state below the glass transition temperature, or by rapid solvent removal from a swollen glassy polymer. High free volume polymers are currently used in industrial membranes for transporting either gases or liquids. The voids in these materials, however, are not interconnected and therefore reflect a low accessible surface area as determined by gas adsorption. Moreover, the pore structure is irregular and not homogeneous.
Another existing class of porous organic materials includes polyacetylenes containing bulky substituent groups. The high gas permeabilities of poly(1-trimethylsilyl-1-propyne) (“PTMSP”) have been observed since 1983. This material contained a large free volume (˜30%), and was able to separate organic compounds from gases or water. The stability of PTMSP is limited by its rapid loss of microporosity from reaction by heat, oxygen, radiation, UV light, non-uniform pore structure, or any combination of the above.
One recent display of porous organic materials is the polymers of intrinsic microporosity (PIMs). These polymers have been reported to contain relatively high surface areas (430-850 m2/g) measured by gas adsorption due to their highly rigid and contorted molecular structures unable to efficiently pack in space. These materials, however, display marked hysteresis at low pressures.
Thermal dehydration of phenyl boronic acids has been known to form six-member boronic acid anhydride (boroxine) rings. Although crystal structures of molecular compounds containing one boroxine ring have been studied, little if anything is known about materials produced from the condensation of multi-topic boronic acids.